fix: allow readVcf() to accept a connection (textConnection, url, etc.) (#28)

[jmg421]

Problem

Users who hold VCF content in memory (e.g. fetched from a database blob, downloaded via url(), or stored as a character vector) had to write to a temp file before calling readVcf(). This round-trip added unnecessary I/O latency, especially for large VCFs (#28).

Fix

• R/AllClasses.R — call setOldClass() for the four common S3 connection subclasses (textConnection, rawConnection, url, file) so that S4 dispatch resolves the "connection" signature before any setMethod() calls.
• R/methods-readVcf.R — add two new setMethod(readVcf, c(file="connection", ...)) dispatches. The implementation reads all lines from the connection into a temp .vcf file (which Rsamtools requires), calls the existing readVcf,character method, and cleans up. The caller's connection is left open or closed depending on whether it was already open before the call.
• man/readVcf-methods.Rd — document the new dispatch, update the file argument description, add \alias entries, and include a short usage example.

Usage

lines <- readLines("my.vcf")
con   <- textConnection(lines)
vcf   <- readVcf(con, "hg19")   # no temp file needed by the caller

Closes #28