Add ARP table fetching

Now possible to fetch ARP table!
Merge pull request #91 from plajjan/arp-table

Add ARP table fetching
Fix fact collection on MX

Use xpath instead of findtext which seem to yield the same result on
previously working platforms (I tried on EX4200) while still working on
MX960. Same xpath is used for single-re platforms so should be all good.

Fixes #94.
View/Table for LDP neighbor data

Fetches complete LDP neighbor information.
Add View/Table support for ISIS adjacencies

Retrieve information from 'show isis adjacencies extensive'.
Add View/Table for BFD sessions

Maps all data available from 'show bfd sessions extensive'.

Key is set to neighbor-address which I'm not sure is correct. The box
I'm working with does not yet support micro-BFD sessions nor do we have
it configured but those would probably not have an IP address associated
with them at all. Similarily, I'm not sure what happens if there are two
BFD sessions to the same address but in different VRFs.
Merge pull request #95 from plajjan/mx-facts

Fix fact collection on MX
Merge pull request #97 from plajjan/collect-ldp-neighbor

View/Table for LDP neighbor data
Merge pull request #98 from plajjan/fetch-isis-adj

Add View/Table support for ISIS adjacencies
Merge pull request #101 from plajjan/collect-bfd-sessions

Add View/Table for BFD sessions
Merge remote-tracking branch 'upstream/master'

merge with original