<rdar://problem/14972424>

- Made the dynamic register context for the GDB remote plug-in inherit from the generic DynamicRegisterInfo to avoid code duplication
- Finished up the target definition python setting stuff.
- Added a new "slice" key/value pair that can specify that a register is part of another register:
    { 'name':'eax', 'set':0, 'bitsize':32, 'encoding':eEncodingUint, 'format':eFormatHex, 'slice': 'rax[31:0]' },
- Added a new "composite" key/value pair that can specify that a register is made up of two or more registers:
    { 'name':'d0', 'set':0, 'bitsize':64 , 'encoding':eEncodingIEEE754, 'format':eFormatFloat, 'composite': ['s1', 's0'] },
- Added a new "invalidate-regs" key/value pair for when a register is modified, it can invalidate other registers:
    { 'name':'cpsr', 'set':0, 'bitsize':32 , 'encoding':eEncodingUint, 'format':eFormatHex, 'invalidate-regs': ['r8', 'r9', 'r10', 'r11', 'r12', 'r13', 'r14', 'r15']},

This now completes the feature that allows a GDB remote target to completely describe itself.