Extra options for predict methods

This bump to version 1.5.5 brings the possibility to only limit the
prediction to only calculate the mean, with the variance and
emualted gradient being now optional. Note that the return is now
always a 3-element tuple, with some of those elements set to `None`
if they're not being calculated.

Closes #13

setup.py

@@ -3,7 +3,7 @@
 from distutils.core import setup
 
 setup(name='gp_emulator',
-      version='1.5.4',
+      version='1.5.5',
       description='A Python GaussianProcess emulator software package',
       classifiers=[
 	'Development Status :: 4 - Beta',