Added some MultiSet functionality

[7h3kk1d]

Default Methods:

• MultiSet#contains plus overload for amount
• MultiSet#union
• MultiSet#difference
• MultiSet#inclusion
• MultiSet#intersection
• MultiSet#symmetricDifference
• MultiSet#merge
• MultiSet.EquivalenceRelations#sameElements

Abstract Methods:

• MultiSet#unique

I feel pretty gross about some of the current Javadocs but I thought it would be easier to discuss over PR.

[jnape]

@7h3kk1d do me a favor and comment on #17 so I can assign it to you