Consolidate documentation into a single location in source.

This system uses `pandoc` to convert Markdown into PDF and HTML
for each release. These currents docs were taken from our Wiki
and README.
Merge pull request #474 from massie/docs

Consolidate documentation into a single location in source.
[ADAM-476] Adding 'childLocators' to FileLocator

Updated the FileLocator trait to include a childLocators method, which
implements the ability to scan the children of a locator (in addition to
finding the parent).  This will be required for the AvroMultiParquetRDD
we're planning to introduce.  Unfortunately (see the comments) this
isn't possible to implement in HTTPFileLocator without additional
assumptions.

Also filled in the implementation of some missing methods in
HTTPFileLocator in this commit, and added comments to FileLocator
overall.
[ADAM-477] Adding HDFSFileLocator implementation

HDFSFileLocator implements the FileLocator API, but uses the Hadoop
interface to find files in an HDFS system.

Includes the InputStreamByteAccess implementation, which is used
internally.
[ADAM-463] Added AvroMultiParquetRDD

AvroMultiParquetRDD takes a FileLocator which indicates the _parent_
directory (in HDFS, or S3) of a set of parquet files, and which loads
_all_ row groups from all the parquet files that are immediate children
of that FileLocator.

(Here, "children" is defined w/r/t the childLocators method
implementation in the corresponding FileLocator.)

(Joint work with Carl Yeksigian.)