CPU bug fixes and Simulator enhancements.

Bug Fixes:

- Fixed several bugs in the CPU that caused processor status flags to
  be set incorrectly.  Instructions affected were: STA, STX, STY, CMP,
  CPX, CPY, BIT.

- Made some internal-use-only methods on the CPU class private.

- Fixed incorrect disassembly of (Indirect,X) and (Indirect),Y
  instructions. Although this didn't affect behavior, it certainly
  caused me some confusion in debugging.

- Added missing "BCS" instruction to instruction table.

Enhancements:

- Now includes a full version of Lee Davison's Enhanced 6502 BASIC
  bundled as source code and a ROM image. Get that REAL COMPUTER
  EXPERIENCE!(tm)

- If a file named "rom.bin" exists in the same directory where the
  simulator is executed, it will be loaded at addresses $d000-$ffff.

- Gave the CPU an idle loop to make simulated timing a little more
  realistic (but this is still an area needing major improvement)

- Changed the CPU's toString() method to give better debugging output.

- Added a small typeahead buffer to the Console.

- Better exception messaging.

Misc:

- Bumped version to 0.5, updated README.

src/main/java/com/loomcom/symon/Bus.java

@@ -42,9 +42,10 @@ public class Bus {
         MemoryRange memRange = device.getMemoryRange();
         for (Device d : devices) {
             if (d.getMemoryRange().overlaps(memRange)) {
-                throw new MemoryRangeException("The device being added " +
-                                               "overlaps with an existing " +
-                                               "device.");
+                throw new MemoryRangeException("The device being added at " +
+                                               String.format("$%04X", memRange.startAddress()) +
+                                               " overlaps with an existing " +
+                                               "device, '" + d + "'");
             }
         }
 
@@ -114,7 +115,7 @@ public class Bus {
                 return d.read(devAddr);
             }
         }
-        throw new MemoryAccessException("Read failed!  No device at address.");
+        throw new MemoryAccessException("Bus read failed. No device at address " + String.format("$%04X", address));
     }
 
     public void write(int address, int value) throws MemoryAccessException {
@@ -127,7 +128,7 @@ public class Bus {
                 return;
             }
         }
-        throw new MemoryAccessException("Write failed!  No device at address.");
+        throw new MemoryAccessException("Bus write failed. No device at address " + String.format("$%04X", address));
     }
 
     public SortedSet<Device> getDevices() {