Add new comparison methods.

Closes #4.

*Example*

>> a = dseries(randn(4,5),'2000Q1')

a is a dseries object:

       | Variable_1 | Variable_2 | Variable_3 | Variable_4 | Variable_5
2000Q1 | 2.4245     | -1.036     | -0.87587   | -0.57001   | -1.6989
2000Q2 | 0.9594     | 1.8779     | 0.31995    | -1.0257    | 0.6076
2000Q3 | -0.31577   | 0.9407     | -0.55829   | -0.90875   | -0.1178
2000Q4 | 0.42862    | 0.78735    | -0.31143   | -0.2099    | 0.69916

>> a.Variable_3>0

ans =

  4x1 logical array

   0
   1
   0
   0

>> a(a.Variable_3>0)

ans is a dseries object:

       | Variable_1 | Variable_2 | Variable_3 | Variable_4 | Variable_5
2000Q2 | 0.9594     | 1.8779     | 0.31995    | -1.0257    | 0.6076