Removed obsolete macros from configure.in, formatted README

I removed a number of obsolete macros from the configure.in; these were mostly instances of `AC_TRY_COMPILE`,
`AC_TRY_LINK`, and `AC_TRY_RUN`. I ran configure before and after making the changes and diffed the configure.log
files -- the output is identical except for the fact that some of the new (non-obsolete) macros #define a few
additional constants.

I also formatted the README using reStructuredText. This makes it easier to read on GitHub, and it can also be
used with the Python `docutils` package to produce high-quality documentation (e.g., for the Mono website).
With the formatting changes, it's even a bit easier to read the file as just plain text.

Finally, I added patterns to the .gitignore to ignore compiled binaries after building projects (e.g., those in
`mcs/`) and to ignore some other temporary files.