Pandera v0.1.3

This release adds a few nifty features to pandera, special thanks to @mastersplinter and @ralbertazzi:

• We now have official documentation! Thanks to @mastersplinter on the work here.
• the Check class now has a groupby argument, which enables the user to assert properties on subsets of the Column of interest. This opens up the possibility to compare the values or aggregates of values of subsets of a column #42.
• the introduction of hypothesis tests through the Hypothesis class, which is a subclass of the Check class. This enables the user to run hypothesis tests on their dataframe as part of a DataFrameSchema definition. Refer to the documentation for more info #43.
• Columns now have a required argument (default = True), where required=False means that the column is optional #23.
• SeriesSchemaBase now has an allow_duplicates argument (default = True) #24
• add informative errors to check_input and check_output decorators 902f199
• DataFrameSchema(..., strict=True) means that all columns in the dataframe need to be specified in the schema columns. #34
• improved error messaging in general.
• improved CI (codecoverage).