Added support for MIPS, as well as a new "-v" flag which enables you to ...

...specify a variant, such as 'debug' or 'release'.

Added support for arbitrary $EXTRA_OPTIONS, which are passed directly to b2/bjam. Sample usage might be as follows:

  EXTRA_OPTIONS="--build-dir=obj --variant=debug" bash -c "./build-android.sh ..."

which would instruct Boost to build outside of the source directory.