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 don't think you should be using a regexp here. I think you should use startsWith with the possibilities:
if( path.startsWith("hdfs://") ...
else if( path.startsWith("http://") || path.startsWith("https://") ) ....

For the HDFS case, I don't think the RE version is buying you anything startsWith doesn't - except that you might have to add startsWith :)

For the CURL case, it's arguably the case that you can save time using the RE. But given that RE2 is currently an optional component - I'd like to either A) agree that RE2 is going to be enabled by default or B) create a version of this code that doesn't use RE2 (ie startsWith above).