Use custom replacement for table_get_column

As explained in the comment MAD-X r4777 fixes the handling of integer
columns in the table_get_column function. As it seems overkill to
require a new MAD-X version for a minor patch, the function is replaced
with a Cython snippet inside cpymad for now.

When we require a newer version of MAD-X sometime in the future, THIS
COMMIT MAY BE REVERTED.

There is one additional benefit to the replacement code though: The
ability to raise exceptions with more detailed descriptions on what went
wrong.
Add support for integer columns in tables

This requires either MAD-X revision 4777 or the code added in 00281a4.
Add madx.chdir to change current working directory
Change directory when doing CALLs by default

I think this will more likely match any user expectations when CALLing
files with CALL instructions using relative pathes inside them.
Enable RPC of functions in arbitrary modules

This was originally thought as a generalization to make 'os' importable
in the remote process in order to perform ChangeDirectory operations.

Now, I decided to add 'chdir' and 'getcwd' to 'cpymad.libmadx' for
convenience, so this commit is not a necessity for the moment. But I
still believe, that it can prove useful for third party tools that need
to perform special setup (e.g. in MadGUI).
Remove obsolete docstring portions

Obseleted due to 38908cc
Hotfix bug in _close_all_but
Remove redundant os.sysconf call

subprocess.py already does exactly that call to try to set MAXFD
Use inherited pipes for IPC instead of STDIO

The main advantage being that STDIO streams can be redirected
independently without much effort.

Furthermore, this is very slightly faster in terms of startup time and
transfer speed when compared to _multiprocessing.Connection.
Put Connection related code into _libmadx_rpc

There are no changes in the actual code.
Minor cleanups
Update recommendation to use *static* library on windows

Currently, we have only one Cython file linking to MAD-X. In this case
static linkage makes the most sense, since one doesn't have to care
about putting the runtime libraries in the correct folders.
Mention --rpath and --madxdir options to set runtime-lib path
Raise OSError when .c source file is missing

This error is much more explicit and the user will probably not need to
consult the online installation instructions anymore.
Merge pull request #50 from pymad/integer-array-columns

support integer arrays in tables
Merge pull request #51 from coldfix/change-cwd

Add function to change directory in the remote process
Merge pull request #53 from coldfix/update-install-instructions

Update install instructions and recommendations
Make Madx.command a MadxCommands instance.

This is a fully backward compatible change.

The advantage is, that Madx.command now gives access to raw variants of
MAD-X commands, e.g.:

>>> madx.command('twiss', ...)
>>> madx.command.twiss(...)

Will just take all parameters, generically create a MAD-X command string
from them and issue them in the interpretor, without any special
treatment whatsoever.
Add more generic function mad_command

The new function should be used in favor of the now deprecated
_mad_command function.
Use the new simplified .command internally

This should be mostly backward compatible. One excepton is .match which
used _mad_command before.