Adding Curl support 

This pull request adds basic support for Curl to Chapel. It provides the following to the user:
* It hooks in with the current I/O system so that the end user can use a "file" Curl handle much as they would a regular file in Chapel
* An interface to the ``curl_easy_setopt`` and ``curl_easy_perform`` functions, as well as functions to read from Curl into a string or a file (and bypass QIO). 
* It changes the make system so that we now use two user defined environment variables to find where the appropriate libraries and includes are for an "extra file system", this will be used in the future to specify paths for all "extra" file systems or I/O plugins.  
 - Note: These variables are named ``CHPL_AUXIO_INCLUDE`` and ``CHPL_AUXIO_LIBS`` since I plan on renaming *auxfilesys* to *auxio* in the near future (since Curl doesn't qualify as a file system).
* It changes the ``open`` function in ``IO.chpl``  so that we now parse the path to decide whether or not we should be using HDFS, Curl, or the local file system. This uses regexp when ``CHPL_REGEXP == re2``, otherwise, we parse the string using ``indexOf`` (etc.). 
* Some example usages can be found at: https://github.com/tzakian/chapel-unit-tests/tree/curl

 @mppf said he would review.
I've got a bad feeling about this.

Seriously though - could you make it so that the C constants are just referred to, like how the error numbers are handled? In other words, use the C integration already in Chapel so you don't have to repeat this block. What if the Curl developers change one of the numbers? Are we even testing all of these? If you make a mistake in only listing their names you get a compile error - but if you have the wrong number it's a run-time error which can be much harder to figure out.