Move code to v2 subfolder for greater compatibility

[nicksnyder]

So projects who are still using Glide or Dep can continue to depend on the latest version of go-i18n.

Related to #179 and #184

[codecov]

Codecov Report

Merging #187 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #187   +/-   ##
======================================
  Coverage    82.9%   82.9%           
======================================
  Files          14      14           
  Lines        1293    1293           
======================================
  Hits         1072    1072           
  Misses        158     158           
  Partials       63      63

Impacted Files	Coverage Δ
v2/internal/plural/operands.go	67.64% <ø> (ø)
v2/i18n/bundle.go	59.61% <ø> (ø)
v2/internal/plural/rules.go	100% <ø> (ø)
v2/i18n/localizer.go	80.43% <ø> (ø)
v2/internal/template.go	90.47% <ø> (ø)
v2/i18n/message_template.go	92.85% <ø> (ø)
v2/i18n/parse.go	73.11% <ø> (ø)
v2/internal/plural/rule.go	100% <ø> (ø)
v2/i18n/message.go	87.71% <ø> (ø)
v2/goi18n/merge_command.go	74.43% <ø> (ø)
... and 4 more

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 25e77af...bc8f1c6. Read the comment docs.

[codecov]

Codecov Report

Merging #187 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #187   +/-   ##
======================================
  Coverage    82.9%   82.9%           
======================================
  Files          14      14           
  Lines        1293    1293           
======================================
  Hits         1072    1072           
  Misses        158     158           
  Partials       63      63

Impacted Files	Coverage Δ
v2/internal/plural/operands.go	67.64% <ø> (ø)
v2/i18n/bundle.go	59.61% <ø> (ø)
v2/internal/plural/rules.go	100% <ø> (ø)
v2/i18n/localizer.go	80.43% <ø> (ø)
v2/internal/template.go	90.47% <ø> (ø)
v2/i18n/message_template.go	92.85% <ø> (ø)
v2/i18n/parse.go	73.11% <ø> (ø)
v2/internal/plural/rule.go	100% <ø> (ø)
v2/i18n/message.go	87.71% <ø> (ø)
v2/goi18n/merge_command.go	74.43% <ø> (ø)
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 25e77af...5d6aad9. Read the comment docs.