add auxGet function, c2hs fix for Mac OSX 10.9

I created an ```auxGet``` function for retrieving arbitrary auxiliary fields. I also added a c2hs fix for Mac OS X 10.9 which fixed the following compilation error:

```
Building samtools-0.2.1.2...
Preprocessing library samtools-0.2.1.2...
c2hs: C header contains errors:

/usr/include/sys/resource.h:349: (column 75) [ERROR]  >>> Syntax error !
  The symbol `=' does not fit here.

cabal: Error: some packages failed to install:
samtools-0.2.1.2 failed during the building phase. The exception was:
ExitFailure 1
```