added a strings/alphabet.h module

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517358]
improved example by adding sequence printing

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517366]
better docs

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517420]
added support for the protein alphabet

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517424]
better Dpxygen typesetting

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517472]
extended the protein alphabet with the letter 'O'

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517493]
IUPAC16 -> DNA_IUPAC renaming

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517872]
readability change

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517875]
fixed comment

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517900]
added alphabet

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517901]
fixed protein wildcard symbol

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18517916]
added RNA alphabets

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518032]
silence VS warnings about missing return statements

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518050]
removed TODOs

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518065]
mentioned RNA alphabets

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518084]
added handling of RNA alphabets in bits_per_symbols()

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518119]
make explicit use of the thrust::transform_iterator

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518131]
use the new Alphabet module

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518180]
- added const qualifiers
- changed examples to use the assign() method

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518240]
fixed example as from the docs

[git-p4: depot-paths = "//sw/gpgpu/nvbio/main/": change = 18518282]