uhd: extended uhd::dict to be able to be used like a Python dictionary

* Both dict["key"] and dict.get("key") syntax work
* Updated uhd_fft, uhd_rx_cfile, and uhd_siggen_base.py to use new syntax
analog: agc3_cc_impl Windows compatibility

* If MSVC is detected, use std::vector instead of variable-length array
Merge remote-tracking branch 'ncorgan/uhd_dict_swig_extend'
Merge remote-tracking branch 'ncorgan/analog_fix_20130926'