Deprecate PageReader's constructor

[dmikurube]

PageReader is now considered as a part of Embulk API. Although calling its constructor has been the only way to construct PageReader from plugins, PageReader should no longer provide its own constructor for plugins in embulk-api. Instead, Exec is going to have Exec.getPageReader.

• v0.10: PageReader's constructor is marked as @Deprecated, and Exec.getPageReader is added. But Embulk v0.9-compatible plugins still have to call the constructor because Embulk v0.9 has only the constructor, no Exec.getPageReader.
• v0.11: PageReader's constructor will still be kept, but plugins should start using Exec.getPageReader instead.
• v1.0: PageReader's constructor would be removed at some early point of v1.0.