Change alphabet conversion dictionaries to arrays.

I get a reasonably good speedup with this for longer nucleotide strings:

Current dict-based `reversecomplement`:

```julia
julia> test_rev_compl(n::Int) = reversecomplement(nt"ATCG"[rand(1:4, n)], DNA_COMPLEMENT)
# method added to generic function test_rev_compl

julia> compare([()->test_rev_compl(100), ()->test_rev_compl(1000), ()->test_rev_compl(10000), ()->test_rev_compl(100000)], 10000)
4x4 DataFrame:
            Function  Elapsed Relative Replications
[1,]    "# function"  0.10635      1.0        10000
[2,]    "# function" 0.821165  7.72134        10000
[3,]    "# function"  7.23151  67.9973        10000
[4,]    "# function"  72.7856  684.397        10000
```

Array-based `reversecomplement`:

```julia
julia> compare([()->test_rev_compl(100), ()->test_rev_compl(1000), ()->test_rev_compl(10000), ()->test_rev_compl(100000)], 10000)
4x4 DataFrame:
            Function   Elapsed Relative Replications
[1,]    "# function" 0.0758033      1.0        10000
[2,]    "# function"  0.511049  6.74177        10000
[3,]    "# function"   4.53903  59.8791        10000
[4,]    "# function"   45.3301  597.996        10000
```

BTW, nice work on this package!

Cheers,

   Kevin